If both are equally available, the bare cost comes down to weight of steel. The tube would be the less expensive. If you compare the cost of fabricating connections, you would have to speak to the steel fabricator. Single plate connections to tubes or W's do not vary greatly in my area. However, if you have moment connections, different shops like to use different connections if the type of connection is left up to the fabricator. This doesn't happen often because there are other constraints which may dictate the type of connection. Anyway, contact your local steel fab/supply shops and ask the same question.